Manufacture of filaments or cathodes for electric lamps, thermionic tubes and the like



Patented a. 27, 1925. U N I T E D S PA ENT OFFICE.

WILLIAM RICHARD BULLIMORE, LONDON, ENGLAND.

MANUFACTURE OF FILAMENTS 0R CATHO'DES FOR ELECTRIC LAMPS, THERMIONJEC TUBES AND THE LIKE.

No Drawin g.

T 0 all whom it may concern:

Be it known that I, WILLIAM RICHARD BULLIMORE, a subject of the King of Great Britain, of Aberdeen Works, Aberdeen Lane, Highbury Grove, London, N. 5. England, have ,invented new and useful Improveme'nts in the Manufacture of Filaments or Cathodes for Electric Lamps,

' Thermionic Tubes and the like (for which I have filed applications in Great Britain No.

of metallic filaments which enables filamentsof comparatively large diameter to be used for a given resistance, thereby obtaining greater mechanical strength and emission surface without increase of temperature or of current consumption,

A further object of the invention is to provide a metallic filament with a thermionically active coating in such a manner that the coating will remain active and united to the filament over a relatively long period without developin any greater temperature than will cause a low, thereby renders ing the invention articularly applicable to the production of laments for employment in thermionic devices of the kind known as dull emitters.

According to the invention an improved process is provided for the manufacture of laments for electric lamps, thermionic tubes and the like which consists in coating a base or refractory metal or allo of relatively hi h specific resistance an melting point wit a noble metal, or alloy thereof, the term noble metal- -being intended to conveyLmetals which. are not tarnishable under ordinary. atmospheric conditions.

The invention further consists in coating a base or core formed as above described,

with one or more pure compounds of the alkaline earth meta series.

In carrying the invention into effect according to the preferred form, a filament or Application filed November 24, 1924. Serial No. 751,939.

base or core for the production of a coated filament is formed by coating a base or refractory metal of relatively high specific resistance and melting point with a noble metal or alloy thereof, for example, ni-

chrome wire (80% Ni. Cr.) may be coated with platinum or a refractory metal such as molybdenum or tungsten may be coated with platinum or palladium.

Where platinum. is the coating required it may be produced by passing the core through a bath containing a solution of'ammonium 'platino chloride or other suitable salt, the wire coated by passage through this.

bath being subsequently heated to cause the platinum to become dissociated from the compound and adhere to the core. Alternatively, the base or refractory metal may be coated whilst in the form of an ingot and subsequently drawn down through dies in the usual manner to form a filament or wire of the required thickness. The coating of a noble metal may be imparted to a wire or ingot by means of cathodal deposition.

A composite filament or wire formed as above described may itself be used as a base or core to receive a coating consisting of an inorganic compound or compounds of the alkaline earth metals preferably the oxides of calcium, barium or strontium, either separately or combined. One method of applying such a coating consists in immersing the wire core in" a bath containing a solution of nitrates of the alkaline earth metals, the coated wire being then dried and raised to a high temperature of about one thousand degrees 0., to reduce the nitrates to the corresponding oxides, but any of the methods for producing a coating of inor anic compounds of the alkaline earth meta s described in my concurrent application No. 28491/23 may be employed whilst the base or core may consist apart from nichrome wire of Hoskins alloy or a refractory metal suchas molybdenum.

By using a core of a metal of highspecific.

resistance, filaments of comparatively large diameter are required for a given resistance thereby, obtaining greater mechanical strength and emission surface under the same conditions of temperature as compared with platinum or other metal used alone. Consequently the same emission may be obtained as heretofore with lower current consumption. The invention further enables mama.

filaments of desirable thickness to be used in constructing cathodes which will require batteries of high voltage and low current capacity, thus enabling dry batteries to be used where accumulators would otherwise be required.

By uniting a thermionically active coating to a metallic base or core as above described a filament is obtained inwhich both the coating and the core have such charac teristics of permanency and specific resistance that the completed filament or cathode will remain thermionically active over a relatively long period without developing any greater temperature than will cause a dull glow. I

I claim:

1. A heated cathode for vacuum tubes, comprising a core of relatively high specific resistance and melting point coated with a noble metal and an active coating of one or more compounds of the alkaline earth metals.

2. A heating cathode for vacuum tubes, comprising a core of relatively high specific resistance and. melting point coated with a noble metal and an active coating of one or more compounds of the alkaline earth metals, said compounds comprising the inorganic compounds of calcium, barium and strontium.

3. A process for the manufacture of cathode filaments for vacuum tubes which c011- sists in forming a core of relatively high specific resistance and melting point, coating said core with a noble metal, coating the filament thus formed with a compound or compounds of the alkaline earth metals in solution or suspension and heating the filament thus coated.

4. A process for the manufacture of cathode filaments for vacuum tubes according to claim 8, including the step of passing the core through a solution or suspension of a compound of a noble metal and heating to cause dissociation of the compound, and adherance of the metal to the core.

5. A heated cathode for vacuum tubes, comprising a core of relatively high specific resistance and melting point coated with a noble metal alloy and an active coating of one or more compounds of the alkaline earth metals.

6. A heating cathode for vacuum tubes, comprising a core of relatively high specific resistance and melting point coated With a noble metal alloy and an active coating of one or more compounds of the alkaline earth metals, said compounds comprising the morganic compounds of calcium, barium and strontium.

In witness whereof I affix my signature.

l/VILLIAM RICHARD BULLIMORE. 

